Validate Binary Search Tree
Description
Given a binary tree, determine if it is a valid binary search tree (BST).
Assume a BST is defined as follows:
The left subtree of a node contains only nodes with keys less than the node's key.
The right subtree of a node contains only nodes with keys greater than the node's key.
Both the left and right subtrees must also be binary search trees.
Example 1:
2
/ \
1 3
Input: [2,1,3]
Output: true
Example 2:
5
/ \
1 4
/ \
3 6
Input: [5,1,4,null,null,3,6]
Output: false
Explanation: The root node's value is 5 but its right child's value is 4.
Solution
class Solution {
public:
bool isValidBST(TreeNode* root) {
if(!root) return true;
return helper(root, numeric_limits<long>::min(), numeric_limits<long>::max());
}
bool helper(TreeNode* root, long minVal, long maxVal) {
bool res = minVal < root->val && root->val < maxVal;
if(root->left) {
res &= helper(root->left, minVal, root->val);
}
if(root->right){
res &= helper(root->right, root->val, maxVal);
}
return res;
}
};
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